1. Field of the Invention
The present invention relates to a method for crystallizing an amorphous silicon film. More particularly, the present invention relates to a method for crystallizing an amorphous silicon film by forming electrodes on the amorphous film, providing a very thin cobalt (Co) thin film between two electrodes, and applying an electrical field across the amorphous silicon film while heating the amorphous silicon film, thereby lowering a crystallization temperature.
2. Discussion of the Related Art
Polycrystalline silicon films have come into widespread use as active regions of thin film transistors in semiconductor devices, especially for liquid crystal displays. The use of polycrystalline silicon in thin film transistors has increased because polycrystalline silicon has lower defect density and higher field effect mobility than amorphous silicon. While polycrystalline silicon is usually formed under high temperature conditions, methods of fabricating polycrystalline silicon thin film transistors (polysilicon TFT) under low temperatures have recently been introduced.
Low temperature polycrystalline silicon (polysilicon) can be manufactured on a relatively large scale using a low processing temperature and can be manufactured to have performance characteristics similar to high temperature polysilicon. Various methods are known for forming low temperature polysilicon such as solid phase crystallization, laser crystallization and the like.
A laser crystallization is a method of crystallizing an amorphous silicon film by thermal treatment applied to the amorphous silicon film using a laser. For example, a low temperature crystallization, as described by Hiroyaki Kuriyama et al., Jpn. J. Phys. 31, 4550 (1992), is performed at 400.degree. C. and provides a crystallized product having excellent performance characteristics. Unfortunately, this method is unsuitable for uniform crystallization and fabrication of polysilicon on large substrates because of its low throughput and the need to employ expensive equipment.
A solid phase crystallization requires thermal treatment of amorphous silicon at 600 to 700.degree. C. for 1 to 24 hours, uses inexpensive equipment, and produces crystals of uniform grain size. However, the method cannot be applied to amorphous silicon formed over glass substrates, due to the method's relatively high temperature and long processing time. This method also has poor yields.
A recently introduced method for crystallizing amorphous silicon at low temperatures is metal induced crystallization (MIC), discussed in M. S. Haquc et al., Appl. Phys. 79, 7529 (1996). The MIC is an excellent method for reducing the temperature of crystallizing amorphous silicon and involves providing a specific kind of metal in contact with amorphous silicon. The metal may be provided as a thin film on the amorphous silicon so that the metal provides nucleation sites over the amorphous film. In the MIC using Ni as the nucleation metal, described in C. Hayzelden et al., J. Appl. Phys. 73, 8279 (1993), NiSi.sub.2 which is the lowest formation energy phase of nickel silicide, forms and acts as a nucleus to accelerate the crystallization of the amorphous silicon. Actually, NiSi.sub.2 has the same lattice structure as silicon and the lattice constant of NiSi.sub.2 is 5.405 .ANG., which is close to the 5.430 .ANG. of silicon. Thus, NiSi.sub.2 nucleates and accelerates crystallization in the &lt;111&gt; direction, as shown in C. Hayzelden et al., Appl. Phys. Lett. 60, 225 (1992). Such a method of MIC is affected by the time and temperature of thermal treatment and the quantity of metal. As the quantity of metal increases, the temperature necessary for the thermal treatment, in general, is reduced.
The MIC has the advantages of increasing the effect of metal induced crystallization proportional to the quantity of metal and decreasing the temperature for low temperature crystallization. On the other hand, the MIC has the disadvantage of changing the intrinsic characteristics of the resulting silicon film due to contamination inside the crystallized silicon film. Moreover, such a method requires a long thermal treatment of 10 hours or more.
A crystallization method using a metal solution to decrease metal contamination caused by the MIC has been proposed. According to this method, the surface of an amorphous silicon film is coated with a metal solution and then the amorphous silicon film is crystallized by metal induced crystallization. This method reduces levels of metal contamination. However, this method has the disadvantage of low crystallization rates.